1. Field of the Invention
This invention relates to a method and apparatus for measuring, preserving or restoring audio to video synchronization, or lip-sync, in television systems which use video processing devices such as frame synchronizers that generate large time delays in a processed video signal.
2. Description of the Prior Art
With the advance of television technology, and in particular the development of complex systems for processing video portions of television signals, time delays of video signals which are passed through these processing systems are increased to a point where a noticeable delay in the video signals, with respect to their associated audio signals, can be generated. One typical situation where this problem may arise is in the transmission of network programming over satellites.
In the past, the network programming in some parts of the U.S. was relayed by microwave links from one station to the next in serial fashion. This system worked but was costly and not always reliable. With the advent of satellites, the programming is now relayed via satellite directly to the television station. This system works better than that of the microwave links, but creates new system problems.
Due to the use of the satellites, the video signal which is received by the station contains what is known as time-base error due to the slow figure-eight drifting pattern of the satellite. This timebase error is a change of the phase of the television signal, with respect to a stable reference, due to the slowly changing transmission path length caused by the drifting satellite. In order to accommodate this time-base error introduced by the satellites transmission, stations use video frame synchronizers to receive the satellite video. The frame synchronizers removes the time-base error from the video, but in the process will cause a varying delay to be added to the video. The delay given to the video signal by the frame synchronizer may change continuously in order to make up for the changing path length of the satellite. In addition, the frame synchronizer may also change the video delay in response to slight frequency differences between the video reference of the transmitting station and the receiving station.
The audio portion of the program is not passed through the frame synchronizer and, therefore, is undelayed with respect to the delayed video leading to a lip-sync error. In addition, the audio may be transmitted over land lines rather than via the satellite thus aggrevating the audio to video timing error due to the addition of the changing satellite delay. Further errors may be accumulated by transmitting the video from the television studio to transmitter site via one path while the audio is fed to the transmitter via another.
Normally, in electronic systems, changes of delay times in associated signals can be compensated for by inserting fixed or manually adjustable delay devices in one of the signal paths. In this instance, the delay device is generally needed in the audio signal path. There are several commercially available devices for delaying audio signals by fixed or manually adjustable amounts; however, in this particular application, because the delay is constantly changing, manually adjustable or fixed delays are unsuitable. Devices known as audio synchronizers which can measure the video delay through a particular video processing device, are also used to correct audio-to-video timing problems. An audio synchronizer system is shown in FIG. 1. Once such device is described in U.S. Pat. No. 4,313,135. These devices operate by measuring the video delay through video devices by means of monitoring both the video input and the video output from the device. An audio delay is then caused to automatically match the video delay, thereby insuring that audio-to-video timing is preserved.
Audio synchronizer devices work well; however, the major drawback is that a connection is needed to both the video input and the video output from a particular video delay. Audio synchronizer devices also cannot account for a changing audio delay since they measure only the delay of the video signal. In the instance where the video input and the video output are removed physically, such as in a satellite or microwave links, it can be very difficult to implement an audio synchronizer in the television system. It would be possible to partially eliminate the problem by encoding a number in the vertical interval of the video signal which represents the amount of delay which the video signal is expected to acquire during transmission; however, this method requires an estimate or apriori knowledge of the delay which is not possible where the delay is constantly changing. This method would also require stripping off the old number and inserting a new number every time the video passes through a new delay.